The present invention relates to a process for the production of optical fiber preforms by vapor-phase axial deposition, and more particularly, to a process for the production of optical fiber preforms having an optimum graded index distribution.
A vapor-phase axial deposition process is among the techniques for producing silica-base glass fibers which when used as low loss fibers for communication.
FIG. 1 of the Drawing diagrammatically illustrates the principles of production of optical fiber preforms by the vapor-phase deposition process. Referring to FIG. 1, a glass material, such as SiCl.sub.4, and a refractive index-controlling material (a dopant), such as GeCl.sub.4, which is used to change the refractive index of the glass material, are discharged from an oxyhydrogen burner 2 placed at a lower end portion of a protective container 1 made of, e.g., quartz or Pyrex glass, and are subjected to a hydrolytic reaction to form fine particles of glass. The thus-formed fine glass particles are deposited on a supporting rod 3 so as to form a predetermined space distribution thereof to form a porous preform 4. Further, the porous preform 4 is sintered and vitrified by heating at a temperature of from 1,450.degree. to 1,700.degree. C. by means of a high temperature heat generator 5 to obtain an optical fiber preform 6. In the production of the optical fiber preform 6 by the vapor-phase axial deposition process, as the operation proceeds, the clearance between a deposition face 7 appearing on the lower end of the porous preform 4 and the oxyhydrogen burner 2 gradually decreases, changing the space distribution of each of the glass material and the dopant material. Therefore, in order to maintain a constant clearance, the production system is designed so that the porous preform 4 moves upward while rotating with the growth thereof. However, it has been difficult to produce a preform having a desired refractive index distribution by merely attempting to maintain a constant clearance between the burner and the deposition face of the preform.